It is common practice to grow compound semiconductor epitaxial layers on a substrate by liquid phase epitaxy (LPE). The basis of LPE growth is the production of supersaturation in a growth solution, such that the deposition of solid material occurs onto the substrate. The temperature of the substrate and the growth solution is used to control deposition of solid material onto the substrate.
In the step cooling technique, the substrate and the growth solution are cooled to a temperature below the saturation temperature of the solution. The substrate is slid under the growth solution and a constant temperature is maintained during the growth period. The growth is terminated by sliding the substrate out of the solution.
In the equilibrium cooling technique, both substrate and growth solution are at the saturation temperature of the growth solution. Growth begins when the substrate is brought into contact with the growth solution and both are cooled at a uniform rate. The growth is terminated by sliding the substrate out of the solution.
The supercooling technique is a combination of the step cooling and equilibrium cooling techniques. The substrate is brought into contact with the solution when both are at a temperature below the saturation temperature of the growth solution. The growth solution and the substrate are further cooled during growth.
Thus both the growth rate and the material composition of the layers are controlled by temperature. Heat generation and removal are very slow processes and so it is generally very difficult to obtain temperature uniformity and responsive temperature variation at the same time using LPE techniques. This limits the quality of the structures produced as well as the flexibility in designing complicated growth recipes for novel epitaxial layer structures. For example, the growth temperature may be either kept constant or lowered but is never raised nor raised and lowered in a varying profile.
U.S. Pat. No. 4,594,128 describes an apparatus and method for the low cost growth of an epitaxial layer on a substrate from a solution. Temperature is used to control the deposition of solid material onto the substrate and therefore this procedure suffers from the problems of slow heat generation and heat removal, and temperature uniformity disclosed above. Gas pressure is used to move growth solution into contact with the substrate.
U.S. Pat. No. 4,315,477 and U.S. Pat. No. 5,375,557 are directed towards the production of mercury cadmium telluride (HgCdTe) epitaxial layers. The growth chambers are maintained at a high pressure to reduce the vaporisation of the components from the growth source. In both cases, the formation of epitaxial layers is induced by reducing the temperature of the growth solution to induce crystallisation on a substrate, encountering the problems discussed above.
It would be advantageous to have an LPE system in which it is easy to control supersaturation and the composition of the layers. In addition, it would be advantageous to be able to implement iterative growth easily and to have greater flexibility in designing the growth process.